An example for such a hydraulic machine is a pressure exchanger which is used in a reverse osmosis system.
Another example of such a hydraulic machine is an axial piston pump or an axial piston motor.
During operation of such a hydraulic machine the cylinder unit, which can be a cylinder block or a cylinder drum, rotates with respect to the port plate. The port plate usually has at least two kidney-shaped openings. In one of these kidney-shaped openings there is a low pressure and in the other one of the kidney-shaped openings is a high pressure. This high pressure acts on the side of the valve plate facing the port plate, i.e. the areas of the valve plate between the through-going openings. When the force generated by this high pressure and the area of the valve plate on which this high pressure acts is too high, the valve plate is lifted off the port plate and leakage occurs which should be avoided.
During operation of such a hydraulic machine approximately half of the cylinders are subjected to high internal pressure. This pressure acts on the pressure area pressing the valve plate against the port plate. However, in order to have as many cylinders as possible in the cylinder unit the cylinders are arranged closely side by side in circumferential direction. This limits the area on which the pressure within the cylinders can act. It is still possible that the force generated by the pressure in the high pressure kidney is larger than the force generated by the pressure in the high-pressure cylinders.